When designing digital telecommunications systems, it is important to find a compromise between transmission reliability and transmission speed. As a function of the nature of the processed data, it may be preferable to transmit with a very low error rate but a likewise low speed for given physical resources, whereas in another case, for the same physical resources, the speed will be favored to the detriment of the error rate, that is to say of the robustness.
Concerning telecommunications systems used to transmit data between aircraft and terrestrial stations, between satellites and aircraft, or systems allowing aircraft to communicate directly with one another, the search for the best speed/robustness compromise must therefore be taken into account.
Telecommunications systems customarily have to send and receive a plurality of data streams, each type of stream having constraints in terms of service quality specific thereto. Moreover, the transmission channel varies in the course of time. Techniques make it possible today to adapt the way in which the digital data are transmitted as a function of the type of data, that is to say of the desired service quality, and of the state of the transmission channel. The estimated properties of the transmission channel and the result of this estimation makes it possible to choose a given mode of transmission for a given stream type while taking account of the state of said channel. A mode of transmission corresponds, for example, to the choice of a digital modulation and to the choice of a coding rate if a channel coder is used.
Furthermore, a way of improving speed and/or reliability is to use techniques exploiting transmission diversity. Today, multi-antenna techniques are being increasingly used, whether in mobile telephony or else to install high-speed wireless Internet access points. These techniques rely on M sending antennas and N receiving antennas and make it possible to use the propagation properties not only related to spatial diversity but also related to temporal diversity by installing notably codes of spatial-temporal type, this technique also being called time-space multiplexing. These techniques are customarily designated by the acronym MIMO, which comes from the expression “Multiple Input Multiple Output”.
Today, the potential of multi-antenna communications for improving the performance of digital communications in terms of robustness and/or increase in speeds is not used within the context of telecommunications systems involving one or more aircraft.